The dynamic range (DR) of a digital image is defined as a ratio of the brightest pixel value with respect to the darkest pixel value in the image. The human visual system (HVS) is capable of processing a much higher dynamic range than typical digital cameras or monitors. Therefore, conventional imaging devices cannot capture or display images as humans perceive them. Accordingly, the demand has increased with regard to processing images having a greater dynamic range than the conventional imaging devices are capable of processing. These images are called high dynamic range (HDR) images.
The technology for generating an HDR image is to expand the dynamic range by combining a plurality of images with different exposure levels. The HDR image may be obtained by combing a number of short exposure images (SEI), which are obtained with a short exposure period based on a bright area, and a number of long exposure images (LEI), which are obtained with a long exposure period based on a dark area. In other words, when there are large differences in brightness between the areas of one image, an overall clear image can be obtained even from the image with large differences in brightness between its areas by capturing images of bright areas with a short exposure period and capturing images of dark areas with a long exposure period.
For such combining of multiple images, a plurality of images needs to be aligned and a difference in brightness at a stitched boundary of two images should be at an appropriate level. Further, the boundary between two different areas should look natural without artifacts. However, when images with noise or active motion are combined, it is very difficult to synthesize the images without decreasing dynamic ranges thereof and causing motion artifacts.
If a subject or a camera moves, the SEI and the LEI are made different and therefore a ghost effect may occur when an HDR image is synthesized. The ghost effect refers to a phenomenon in which, due to motion of a subject in a plurality of images, when the images are merged or stitched, the images are overlapped according to the motion of a subject.
To overcome the issue of the ghost effect as above, Korean Patent Application Publication No. 10-2011-0136152 published on Dec. 21, 2011, entitled “Apparatus and method creating ghost-free high dynamic range image (HDRI) using filtering” is disclosed.
The prior art above relates to an apparatus and method for creating ghost-free HDRI based on filtering, where a ghost area is not specifically determined in an image but the ghost area is defined using a probability based on a global transfer function that is a relationship function of intensities of a plurality of frames, and thereby prevents a ghost effect from occurring when a single HDRI is created from a plurality of low dynamic range images (LDRIs).
However, the prior art above has a problem that the calculation speed for extracting an HDR image is slowed down because a large number of exposure images are required to calculate the probability functions between consecutive exposure images in order to remove the ghost area.